Circulators are well known in the prior art. A circulator is an active or passive three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted only to the next port in rotation.
FIG. 1 shows a prior art three port active circulator. The port numbers are labeled and as can be seen the direction of rotation (referred to in the immediately preceding paragraph) is counter clockwise. In this prior art circulator the drain bias voltages (Vdd) are directly connected to three drain resistors Rd. So the drain currents flow thought the three drain resistors and they are grounded to Vss through common ground resistor Rg. In this configuration, there are significant voltage drops through the various resistors causing DC power consumption to occur and DC power consumption is certainly not desirable in an energy efficient system if it cannot be avoided.
Typically, in active circulators, each drain resistor Rd takes a high value in order to block RF signal and the common ground resistor Rg is also not of a small value for the same reason. Otherwise, even more significant RF power loss will occur across the drain resistors Rd and common ground resistor Rg. Beside DC and RF power loss, the DC voltage drop across the Rd and common ground resistor Rg also limit maximum RF voltage swing across each FET transistor, since applied DC voltage across each FET transistor is reduced by the voltage drop across these resistors. Limiting the RF voltage swing leads to a limitation on the maximum circulator power handling capability. In this prior art configuration, the drain bias voltages (Vdd) are directly connected to the drain resistors so the drain current flow is through the drain resistors Rd and it is grounded through the common ground resistor Rg. Thus there are significant voltage drops through the resistors causing DC power consumption, which is not desirable in an energy efficient system.
Also in FIG. 1 there are three other resistors Rf (bypassed with the capacitors Cf) which form RF feedback networks. These RF feedback networks are typically used in an active circulator in order to adjust transconductance of the FETs. Through this resistor and capacitor network, there is preferably no DC current flowing, so it should not produce a DC power loss. The resistance value of Rf is usually high, so very small RF power is fed-backed from drain so gate, not producing a significant RF power loss either. The feed-back capacitor Cf is very big, to it is functions as a RF bypass capacitor with DC blocking. It controls each FETs RF trans-conductance. Active circulator operation is very sensitive to RF transconductance and depending on trans-conductance, loss and isolation varies.
So if the prior art drain resistors Rd and common ground resistor Rg of FIG. 1 could be eliminated, then the circulator might (1) handle more power and (2) do so in a more energy efficient manner.
Circulators have numerous uses. For example, one port of a three port circulator may be connected to an antenna, while a receiver is connected to a second port of the circulator to receive signals received by the antenna and with a transmitter connected to a third port of the circulator to supply transmit signals to the antenna (with the transmit signals being isolated by the circulator from the receiver which might otherwise be damaged by those transmit signals).
The prior art includes: (i) “Active Circulators—The Realization of Circulators using Transistors” S. Tanaka, N. Shimomura, K. Ohtake, Proceeding of the IEEE, March. Vol. 53, Issue: 3, pages: 260˜267, 1965; (ii) “A 1.5-9.6 GHz Monolithic Active Quasi-Circulator in 0.18 um CMOS Technology” Shih-Chieh Shin, Jhih-Yu Huang, Kun You Lin and Huei Wang, IEEE Microwave and Wireless Components letters, Vol. 18, No 12 Dec. 2008; and (iii) “GaAs Monolithic Implementation of Active Circulator”, Mark A. Smith, IEEE Microwave Symposium 1988.